In conventional drilling a drill string is used including a drill bit at its lower end, and progression into the earth is obtained by rotating the drill string while putting weight on the bit. In order to be able to perform directional drilling, a specialised so-called bottom hole assembly has to be used, which forms the lower part of the drill string. At minimum, in order to be suitable for directional drilling, a bottom hole assembly must comprise a drill bit, a drill steering system, and a surveying system. The drill bit forms the lower end of the drill string and is provided with cutting elements for progression into the earth formation. The drill steering system serves to point or push the drill bit into the desired direction. For this purpose, two different approaches are currently used, on the one hand rotary steering systems wherein the rotation of the drill bit is deflected into the desired direction while the entire drill string is rotated from surface, or mud motors in combination with bent subs or housings, wherein only the lower end of the drill string is rotated by the action of the mud motor. The surveying system can include a measurement-while-drilling (MWD) system and/or a logging-while drilling (LWD) system for determining orientation parameters in the course of the drilling operation and/or measuring parameters of the formation or in the borehole.
Directional drilling operations are becoming more and more important for the optimised production of oil or gas from subsurface formations. An example are so called ‘Extended Reach’ wells, which are wells which typically laterally extend up to 2 kilometers or more from the wellhead, with high angle or horizontal deviation. During drilling of such a borehole a plurality of situations and problems can be encountered which can require specialized equipment and tools at the lower end of the drill string to deal with. If the need for such specialised equipment is known in advance, it can sometimes be included in the bottom hole assembly. For example, the surveying system can include highly specialised logging tools for surveying a particular parameter of the surrounding formation or inside the borehole.
In this way, bottom hole assemblies used for directional drilling have developed to a high degree of complexity. Due to the high cost thereof, the risk of a loss of the bottom hole assembly in the borehole increases significantly. Also, it is impossible to include all equipment needed in unforeseen situations. For example, when suddenly mud losses are encountered, it may be desirable to seal fluid communication between borehole and the surrounding formation near the drill bit, but normally this cannot be done with the bottom hole assembly in place.
It is normally undesirable to pull the entire drill string up to surface, in order to replace the bottom hole assembly by, for example, a fluid injection tool (e.g. a cementing tool or a tool for injecting lost circulation material), or in general any other auxiliary tool. Pulling and running back in the drill string can be extremely time-consuming, in an extended reach well this can be a matter of several days. In some critical situations pulling the entire drill string up to surface is not be practically performable at all.